As will be understood from the following description, the present invention was developed for increasing the availability of precision approach landings using a Global Positioning System (GPS) Landing System (GLS) and an Inertial Reference System (IRS) at airports anywhere in the world.
The Automatic Landing Systems (i.e., autopilots) on today's commercial airplanes receive their guidance from a ground-based Instrument Landing System (ILS). In low weather minimums, the integrity and continuity of the ILS transmissions are absolutely crucial to the safety of the airplane during the final phase of approach, touchdown and roll-out. ("Integrity" is the probability that the signals are not hazardously misleading. "Continuity" is the probability that the signals remain present and usable during the approach). The integrity is assured by a set of near-field and far-field monitors, ready to shut down the ILS should the ILS signals move outside allowed tolerances. The continuity of the signals is assured by a backup transmitter. The backup transmitter comes on-line if the primary transmitter fails or is shut down. A key feature of today's systems is that the ground station has the sole responsibility for ensuring the integrity and continuity of its own transmissions. Because ILS equipment is costly due to initial purchase price and maintenance costs, ILSs are only practical at airfields that have large incomes generated by commercial traffic or government funding. Also, ILS signals are sensitive to local building construction and even vehicle movement. This sensitivity increases operating costs, because the ILS operators, such as the Federal Aviation Administration (FAA) in the US, must continually ensure each ILS is producing an accurate signal. Therefore, global implementation is not practical.
The GLS has been proposed as a replacement for ILS. GLS is attractive, because satellite signals are present everywhere in the world, at no cost to airports or other authorities responsible for providing airplane approach information.
In present GLSs, airplane position signals, determined from GPS signals sent by orbiting satellites, are augmented in the airplane by differential corrections (differential GPS) received from a local ground station. The differentially corrected GPS signals are referenced to an intended approach path received by the airplane from the same ground station. The ground station is also responsible for monitoring each satellite and providing airplanes with the integrity status of each satellite. The integrity and continuity of the received airplane position signals depend on the number of satellites in the airplane's field of view, the satellites' positions in the sky (their "geometry"), and the data received from the ground station. The airplane's on-board equipment must determine that the signals being received from satellites and ground station will provide a level of integrity and continuity compatible with the prevailing approach weather minimum for the duration of the approach about to be performed. There will be times and places in the world where the satellites in view cannot support the required continuity and integrity for certain approaches, such as FAA Category 3 (Cat. 3) approaches.
Even when the satellite geometry supports the required continuity and integrity, the signals received by airplanes are subject to environmental threats, such as electromagnetic interference (EMI) (both accidental and malicious), lightning and ionospheric scintillation (i.e., brown-outs associated with sunspot activity). There is also the threat of random satellite failures and satellites setting over the horizon. These threats can affect the reception of some or all of the available satellite signals, resulting in degradation or loss of guidance. Some of the threats are not well understood, and will remain so for several years.
Several methods of enhancing GLS for providing acceptable signals for Cat. 3B and 3C (autoland) approaches have been proposed. One method is to enhance the satellite constellation by making use of another country's satellite system, such as the Russian GLONASS system. This approach places an added burden on the airborne equipment and has complex political implications. Another method uses so-called "pseudolites," ground-based transmitters, located on or near the airport, which mimic satellites by providing additional range information to the airplane. Similar to ILS, this approach is impractical, because it entails large equipment expenditures and maintenance costs in addition to those of the differential GPS ground station. Also, neither of these approaches adequately addresses the environmental and other threats described above, which may produce unreliable GLS data for an indefinite period of time.
Accordingly, a need exists for a low-cost, low-maintenance, worldwide useful, airplane precision approach guidance system that is highly accurate and reliable. The present invention combines the best features of GLS and IRS to fulfill this need.